1. Field of the Invention
The present invention relates to a method and to a device for dosing fuel which is to be injected into a combustion chamber of an internal combustion engine, with a fuel quantity being divided into at least one pilot injection quantity and a main injection quantity, and with respective activation duration corrective values of an injection device for the injection of the pilot injection quantity and the main injection quantity defined.
2. Description of Related Art
Although the present invention and the problem on which it is based are explained below on the basis of common-rail injection systems of passenger vehicle diesel engines, the invention is not restricted thereto, but rather can be applied generally to any desired internal combustion engine.
A common-rail injection system has a common high-pressure fuel accumulator with corresponding outlets for supplying the individual cylinders of an internal combustion engine with fuel. The supply to the cylinders is controlled by means of so-called injectors which are realized for example in the form of piezoelectrically or electromagnetically activated valves. Short paths which can be realized between the injection location and the injector result in low pressure rise times, which benefits the combustion process and its control. It is possible to realize different injection methods, for example a pilot injection, a main injection, and a post-injection, which may be freely programmed by means of modern control units. Pilot injections serve in particular for pre-conditioning of the combustion chamber with regard to temperature and oxygen proportion.
Future emissions standards will require pilot injection quantities which are less than 0.8 mg/injection stroke and which occur in particular in the part-load range. Modern-day fuel dosing concepts are however suitable only for injection quantities which are greater than 0.8 mg/injection stroke.
The injection quantities of the injectors are subject in particular to aging effects which can lead both to an increase and also to a decrease in the injection quantity for a constant activation time of the injector. There are no cost-effective measuring systems for injection quantities which are less than 0.8 mg/injection stroke. Here, too, modern measuring methods are slow and are not sufficiently precise for such small injection quantities.
Furthermore, variance in the injector characteristic variables on account of production tolerances adversely affects dosing accuracy. A reduction in production tolerances of said type will dramatically increase the costs for the injectors.
Said aging drift of the injection quantity for a constant activation duration is compensated nowadays only by means of indirect methods which cannot distinguish between an actual aging process and secondary effects of the drive chain or of the internal combustion engine.
FIG. 5 is an illustration of the dependency of particle emissions PM and NOx emissions for different pilot injection quantities and different λ values.
In FIG. 5, the NOx emissions are plotted on the x-axis and the particle emissions PM are plotted on the y-axis. The curve K2 shows an operating point OP2 for an optimum setting of a pilot injection quantity at a base value, for example, 0.8 mg/injection stroke. Corresponding to the operating state of the internal combustion engine, the λ-EGR control (EGR: exhaust-gas recirculation) moves the operating point OP2 along the curve K2.
If the pilot injection quantity now changes, this leads to a shift to an operating point OP1 on the curve K1 in the case of an excessively large pilot injection quantity and to a shift to an operating point OP3 on the curve K3 in the case of an excessively small pilot injection quantity. In particular, a shift to an excessively large pilot injection quantity brings about an increase in emissions and particle output.
A shift to relatively low pilot injection quantities of less than 0.3 mg/injection stroke may even make combustion, and therefore pre-conditioning of the combustion chamber, no longer be carried out.
EP 1 712 768 A2 discloses a method and a device for controlling the fuel dosing in at least one combustion chamber of an internal combustion engine, with an activation variable which defines the injected fuel quantity being predefined on the basis of at least one characteristic operating variable. A corrective value for correcting the activation variable is predefined on the basis of a variable which characterizes the cylinder pressure.
WO 2007/006660 A1 describes a method for dosing fuel in a combustion chamber of an internal combustion engine, in which method a fuel quantity which is to be dosed for a combustion is dosed by means of a pilot injection and at least one further partial injection, and in which method, during operation of the internal combustion engine, quantity errors in the pilot injection are determined from the signal of a bodyshell sensor. A corrective value is formed as a function of the quantity error of the pilot injection, and at least one of the further partial injections is corrected using the corrective value. Said method permits the derivation of control parameters for the pilot injection quantity of the order of magnitude of 1 mg/injection stroke. If no combustion whatsoever occurs at excessively small pilot injection quantities, said method is equally blind, and thus cannot define a corrective value.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.